Bentonire (smectite) clays are used widely in the construction of liners for hazardous waste landfills, slurry walls, industrial waste treatment lagoons, sewage lagoons, and tank farms. The utility of clays as waterproofing barriers or liners is derived from their ability to disaggregate upon hydration and form a dispersed phase of very small particles. These small clay particles effectively fill the void spaces between larger soil particles resulting in greatly reduced hydraulic conductivity. Thus, the primary function of clay liners, as well as synthetic geo-membraines, is to impede the movement of water.
Smectite clays contain a net negative charge due to isomorphous substitution in the aluminosilicate layers. In nature, this charge is neutralized by inorganic cations such as Na.sup.+ or Ca.sup.2+ on the clay interlayers and external surfaces. Hydration of these cations in the presence of water initiates a separation of the clay layers causing a swelling of the clay. In smectites exchanged with monovalent cations having high hydration energies, e.g., Na.sup.+ or Li.sup.+, the individual clay platelets may become completely separated in the presence of water. However, the maximum distance between individual clay layers of divalent cation-exchanged smectites, e.g., Ca.sup.2+, Mg.sup.2+, is about 19 Angsttoms. Thus, in the construction of clay liners, Na-smectites are more effective in reducing hydraulic conductivity because they form small highly-dispersed particles in water.
The hydration of naturally occurring metal exchange ions on clays also imparts a hydrophilic nature to the clay surfaces. As a result, natural clays are ineffective in removing nonionic organic contaminants (NOCs) from water. However, by simple ion-exchange reactions, the naturally occurring inorganic exchange ions of clays can be replaced by a variety of organic cations and this may change the clay surface from hydrophilic to organophilic. These ion-exchange reactions can be used to form stable organo-clay complexes with high affinities for organic contaminants. Such organo-clays can be used in conjunction with conventional clays to increase the containment capabilities of clay barriers by immobilizing organic contaminants present in leachate. The sorptive properties of soils for NOCs also can be greatly enhanced by organic cation exchange of soil clays. Other possible environmental applications of organo-clays are in the stabilization/solidification of industrial wastes and in water purification.
It is possible to modify the surface properties of clays greatly by replacing natural inorganic exchange cations by larger alkylammonium ions. These ions act as `pillars` which hold the aluminosilicate sheets permanently apart. In the modified form, the clay surface may become organophilic and interact strongly with organic vapours and with organic compounds dissolved in water. These organo-clays are now able to sorb alkanes and aliphatic alcohols to remove organic contaminants from water, and to serve as chromatographic stationary phases.
Until recently, the literature on the sorptive behaviour of organo-clays has been concerned almost exclusively with the organic vapour uptake by dry modified-clay samples. Mortland et al, 1986, Clays and Clay Minerals 34, 581, have studied the uptake of phenol and chlorophenols from aqueous solution by smectites whose cations were exchanged by quaternary ammonium ions of the form [(CH.sub.3).sub.3 -NR].sup.+ where R is an alkyl group. In general, it was found that where R was a large non-polar alkyl group (e.g., R=C.sub.16 H.sub.33) the modified clay samples exhibited greatly improved sorption capacities in comparison with unmodified clays or modified clays in which the exchanged organic ions were small in size. It was also found that smectite exchanged with a small tetramethylammonium ion (herein referred to as TMA-smectite) exhibited much higher affinity for benzene from water than for less water-soluble and large sized 1,2,4-trichlorobenzene, McBride, et al., 1977, in Fate of Pollutants in the Air and Water Environment, Part 1, Vol. 8, pp. 145-154. The extent of benzene uptake by the TMA-smectite was also much greater than by clays exchanged with tetraethylammonium ion (TEA), or with hexadecyltrimethylammonium ion (HDTMA), in the sequence of TMA-smectite &gt;HDTMA-smectite &gt;TEA-smectite. These studies indicated that the exchanged organic ions affected the sorptive behaviour of clay in some manner that appeared to be related to the size and molecular arrangement of the exchanged ion in the clay.
The sorption characteristics of benzene and trichloroethylene (TCE) on HDTMA-smectite were studied from both aqueous solution and the vapour phase, Boyd et al., Soil Science Society of America Journal, 1988, 52, 652. It was found that the dry HDTMA-smectite behaved as a dual sorbent, in much the same fashion as dry soil, in which the bare mineral surfaces function as a solid absorbent and the exchanged HDTMA organic ions function as a partition medium. In aqueous solution, adsorption of non-ionic organic compounds by free mineral surfaces is minimized by the strong competitive adsorption of water, and the uptake of organic solutes by the modified clay is effected mainly by solute partitioning in the organic medium that is formed by conglomeration of large C.sub.16 alkyl groups associated with HDTMA. The presumed partition effect with the HDTMA-smectite is supported by the linear sorption isotherm, lack of a competitive effect between organic solutes, and the dependence of the sorption capacity on the amount of HDTMA in clay. The uptake of organic vapours by dry HDTMA-smectite was greater than by water-saturated HDTMA-smectite because of concurrent adsorption on mineral surfaces, and consequently, vapour uptake isotherms were not linear. The improved sorption of benzene and TCE from water by HDTMA-smectite over that by pure clays was attributed to partition into the highly non-polar hydrocarbon medium.
Improvement of the sorption capacity of soils with low organic matter contents was similarly achieved by cation exchange reactions with HDTMA ions. This study also demonstrated that HDTMA-derived organic matter added to soil was 10-30 times more effective on a unit weight basis than natural soil organic matter for removing organic contaminants from water. A more detailed analysis of this phenomena appears at Environ. Science Technol., 1989, Vol 23 pg. 1365-1370, hereby incorporated by reference.
The organo clays (organo-smectites, and illites and vermiculites) that include a large alkyl quaternary ammonium hydrocarbon radical, for example, hexadecyltrimethylammonium (HDTMA) appear to form partition phases, fixed on the clay surfaces or interlayers, that are derived from the large alkyl hydrocarbon moleties bonded to the quaternary ammonium nitrogen atom. These partition phases are compositionally and functionally similar to bulk phase hydrocarbon solvents such as hexane or octonol. The sorption of organic solutes from water by these organo-clays show characteristics of solute partitioning including linear isotherms, inverse dependence of the sorption coefficient on the water solubility of the solute, and correspondence between the organic matter normalized sorption coefficients (K.sub.om) and the octanol-water partition coefficients. The effectiveness of HDTMA-clays can be increased by using clays with high cation-exchange capacities and high surface charge densities.
It has been discovered that the characteristics of NOC sorption from water by smectite clays exchanged with tetramethylammonium ions were completely different from the partition behavior of clays exchanged with large organic cations such as HDTMA. Due to their small size, TMA ions exist as discrete entites on the smectite layers, and therefore do not form partition phases. Rather, TMA-smectite behaves as a surface adsorbent for aliphatic and aromatic hydrocarbons, such as benzene and substituted benzenes and this is manifested in nonlinear sorption isotherms and strong competitive effects in binary solute mixtures. J. F. Lee. M. M. Mortland, C. T. Chiou, S. A. Boyd. "Shape selective adsorption of aromatic molecules from water by tetramethylammonium-smectite." J. Chem. Soc., Faraday Trans. 1, 85, 2953(1989); and J. F. Lee, M. M. Mortland, C. T. Chiou, D. E. Kile, S. A. Boyd. "Adsorption of benzene, toluene, and xylene by two tetramethylammonium-smectites having different charge densities." Clays and Clay Minerals. 38, 113 (1990), both hereby incorporated by reference. Tetramethylammonium smectite was shown to be an especially effective adsorbent for removing benzene from water, and for removing benzene vapors from air, exhibiting greater uptake of benzene than HDTMA-smectite. However, TMA-smectite also displayed strong shape selectivity resulting in progressively lower uptake of larger aromatic molecules such as toluene, xylene and ethylbenzene from water, but did not show selectivity for removing organic vapors from air.
Tetramethylammonium-smectite has been studied for removal of organic contaminants from water and for chromatographic use in determining the presence and/or concentration of organic contaminants in air, but has not been disclosed as practically useful for removing hydrocarbon contaminants from gases such as air.
Trimethylphenylammonium-smectite has been studied for removing phenol and chlorophenols from water with very limited success (see M. M. Mortland et al., Clays and Clay Minerals. 34, 581 (1986). It is theorized that the ineffectiveness of trimethylphenylammonium-smectite for removal of phenol-based contaminants was due to the relatively high ionization potential of the phenol 0H moiety leaving a negatively charged phenolate molecule that cannot be absorbed or adsorbed sufficiently by the negatively charged clay molecules.
Unexpectedly, it has been found that ion-exchangable clays such as the montmorillonites or smectites, particularly the bentonites that have sodium, potassium, lithium magnesium or calcium as their predominant exchangable cations; as well as hectorite; saponite; nontronite; attapulgite; illitc; zeolites; vermiculite, and the like, that are ion-exchanged with a) a tetra-short chain alkyl quaternary ammonium compound, e.g. tetramethyl; ion-exchanged with b) a quaternary ammonium compound having two or three short chain alkyl moieties, and one or two mono- substituted or unsubstituted cycloalkyl, or a mono- substituted or unsubstituted aryl or alkaryl moieties, e.g. benzyl or phenyl; c) alkylated diazobicyco ions, such as 1,4-diazobicyclo [2.2.2.] (DABCO); or alkyldiammonium cations, such as decyltrimethydiammonium (DTMA) ions, effectively adsorb or otherwise remove aliphatic and aromatic hydrocarbon contaminants from gases, e.g. air, and from water, provided that when the contaminants are removed from water, the contaminants have a pKa in aqueous solution (negative log of the acid dissociation constant) of at least about 10.